Known types of radar apparatus include a pulse radar type and a FM-CW type. The pulse radar type transmits a pulse-like electromagnetic wave to an object to be measured and measures a time until the electromagnetic wave returns after having been reflected by the object to be measured, thereby calculating a distance to the object to be measured.
The FM-CW type generates a beat signal by mixing transmission signals and reception signals whose frequency increase gradually, determines frequencies (i.e., beat frequencies) on a section basis wherein the sections include increasing sections where the frequencies of the transmission signals increase and decreasing sections where the frequencies of the transmission signals decrease, and measures a distance or a relative velocity with respect to the object to be measured based on the beat frequencies in the increasing sections and the beat frequencies in the decreasing sections.
Further, a mobile object detecting apparatus is known which detects a movement of an object or the presence or absence of a mobile object (merely referred to as “the movement of an object” hereinafter) using a standing wave (stationary wave). The standing wave is generated when different electromagnetic waves (traveling waves), which have the same frequency and different traveling directions, overlap one another. The mobile object detecting apparatus, which utilizes the standing wave, generates the standing wave by overlaying the electromagnetic wave radiated by an antenna on the electromagnetic wave returning after having been reflected by the object. The mobile object detecting apparatus detects the movement of the object by utilizing the fact that amplitude of the standing wave varies with the movement of the object (with the variation in distance between the mobile object detecting apparatus and the object).
The mobile object detecting apparatus, which utilizes the standing wave to detect the movement of the object, can detect the movement of the object in a short distance with high precision in comparison with the pulse radar type and the FM-CW type. Thus, it is suitably used in intrusion sensors for determining whether there is an intruder in a cabin of an automobile or a home, sensors for detecting actions of a driver of an automobile, sensors for detecting a heart beat, breathing, a body movement, etc., of a human, etc. It is noted that in such a mobile object detecting apparatus, it is also possible to measure the distance to the object by analyzing variations in amplitude when the frequency of the electromagnetic wave is varied.
FIG. 1 is a diagram for illustrating a situation in which an antenna of the mobile object detecting apparatus radiates a transmission wave and the reflected wave returns to the antenna after having been reflected by the object. In FIG. 1, AT indicates the antenna, OB indicates the object, α indicates the transmission wave, and β indicates the reflected wave.
The standing wave is formed of a combination of the transmission wave α and the reflected wave β. In the following, the standing wave is indicated by γ. Further, it is assumed here that there is no attenuation of the reflected wave β. The amplitude of the standing wave γ becomes 0 when the distance between the antenna AT and the object OB is N×λ/4, where N is a positive integer and even number. FIG. 2 is a diagram for illustrating a situation in which the amplitude of the standing wave γ becomes 0. The amplitude of the standing wave γ becomes its maximum when the distance between the antenna AT and the object OB is M×λ/4, where M is a positive integer and odd number. FIG. 3 is a diagram for illustrating a situation in which the amplitude of the standing wave γ becomes its maximum.
In this way, the standing wave generated between the object and the antenna by radiating the electromagnetic wave from the antenna varies periodically with the distance between the antenna and the object. Thus, by monitoring the variation in the amplitude, it is possible to detect the movement of the object.
JP 2002-357656 A discloses a measurement apparatus which measures a distance to an object to be measured by detecting the standing wave. According to the apparatus, detection means are provided closer to the object to be measured than transmission means for transmitting the electromagnetic wave. The distance to the object to be measured is measured based on a detection signal function formed from a frequency of the electromagnetic wave emitted by the transmission means and the amplitude of the standing wave detected by the detection means.
JP 2007-170990 A discloses an apparatus directed to precisely detect a small movement of the object although it does not use the standing wave. According to the apparatus, the frequency of the transmission signal and the reception signal are converted to a lower frequency using a local signal with a predetermined frequency and a movement status of the subject to be detected is determined based on a difference in a phase between the transmission signal and the reception signal.
However, according to the apparatus which utilizes the standing wave to detect the movement of the object, there may be cases where the movement of the object cannot be detected precisely if the object is located near antinodes or nodes in which the variation in the amplitude of the standing wave with respect to the variation in the distance to the object becomes smaller. The antinodes of the standing wave are points in which the amplitude is maximum and the nodes of the standing wave are points in which the amplitude is minimum. In other words, sensitivity of the apparatus becomes lower if the distance between the apparatus and the object has a predetermined relationship.
FIG. 4 is a diagram for explaining the variation in the amplitude of the standing wave with respect to the same displacement. In FIG. 4, a lateral axis indicates the distance with respect to the antenna. As illustrated in FIG. 4, if the object is located at a point X1 corresponding to the node of the standing wave, or if object is located at a point X3 corresponding to the antinode of the standing wave, the amplitude variation with respect to the replacement of the object (i.e., the variation in the distance to the antenna) becomes minimum. Thus, detection sensitivity for the movement of the object located at the point X1 or X3 becomes low. On the other hand, if the object is located at a point X2 corresponding to a midpoint between the node and the antinode of the standing wave, the amplitude variation with respect to the replacement of the object becomes maximum. Thus, detection sensitivity for the movement of the object located at the point X2 becomes high. It is noted that in FIG. 4, N0, N1, N2 are positive even integers and M1, M2 are positive odd integers.
In this way, according to the apparatus which utilizes the standing wave to detect the movement of the object, high sensitivity regions and low sensitivity regions appear alternately at a λ/8 interval as a distance from the apparatus. λ is a wavelength of the transmission wave, as described above. In particular, if the size of the object located in the low sensitivity region is small, an influence of the low sensitivity becomes larger and thus there is a probability that the movement of the object is not detected.
FIG. 5 is a diagram for illustrating a distribution of the high sensitivity regions and the low sensitivity regions generated in the mobile object detecting apparatus utilizing the standing wave, and the object which meets a specific requirement when it becomes difficult to be detected. As illustrated in FIG. 5, and as described with reference to FIG. 4, the high sensitivity regions and the low sensitivity regions of the mobile object detecting apparatus appear alternately and concentrically centered at the antenna AT. A separation between a center portion H of the high sensitivity regions and center portions L1, L2 (L1 corresponds to the node of the standing wave, and L2 corresponds to the antinode of the standing wave) of the low sensitivity regions is λ/8.
Since the size of the object OB1 in FIG. 5 is sufficiently large, the object OB1 has a portion located in the high sensitivity region. Consequently, the displacement of the object OB1 is detected with high sensitivity by the mobile object detecting apparatus, and thus likelihood that the movement of the object OB1 is not detected becomes low.
On the other hand, the object OB2 in the FIG. 5 has a smaller size, and is located as a whole in the low sensitivity region. In this case, if the displacement of the object OB2 is smaller than λ/8, the displacement of the object OB2 becomes difficult to be detected by the mobile object detecting apparatus, and thus likelihood that the movement of the object OB2 is not detected becomes high.
With respect to such a problem, in JP 2007-170990 A, the following is described. [According to a near-sinusoidal standing wave, the amplitude variation of the standing wave near the amplitude maximum point is small, and thus detection of the maximum amplitude of the standing wave may involve an error. This detection error of the maximum amplitude directly leads to the detection error of a minute movement amount. Thus, in prior art, there is a problem that the accuracy of the detection of the minute movement amount becomes lower because of the detection error of the maximum amplitude of the standing wave.].
In order to solve the problem, it can be contemplated that a relationship between the distance from the apparatus to the object and the wave length of the standing wave is modified by tuning the frequency (wave length) or a position of the antenna. However, tuning of the frequency or the antenna position requires professional technique and thus cannot be performed easily by end users. Further, if the position of the object is not known in advance, it is not possible to use this technique.